The present invention relates to a process and apparatus for measuring the cardiac capacity, and more particularly the cardiac stasis, i.e. the quantity of blood in the heart between one heart beat and the next.
Dilution methods and more particularly those using stains or the injection of a cold liquid (thermodilution) are used for measuring the cardiac output. This output can be given by the following formula: ##EQU2## IN WHICH: D is the average circulation rate,
Q.sub.i is the total quantity of the indicator or marker injected above the hear (stain or frigories), PA1 .DELTA.(t) is the concentration of the indicator or the temperature variation at the outlet from the heart as a function of the time t, basing these times on the moment of starting the injection of the indicator or a previous time.
In connection with measuring the cardiac capacity (ventricular capacity, auricular capacity or total capacity) attempts have been made using more particularly two processes:
The first consists of comparing the curve .DELTA.(t) after the end of injection with a decreasing exponential and deducing therefrom the ventricular capacity. However, either the indicator is injected into the ventricle, in which case the lack of homogeneity of the mixture leads to considerable errors, or the injection takes place into the auricle or above it, in which case it can be shown that the decreasing curve .DELTA.(t) is the sum of two distinct exponentials and that the comparison with one exponential leads to considerable errors.
The second known process is based on the following argument:
Assuming that at time t.sub.o a certain quantity Q.sub.i of the indicator is instantaneously injected, the curve .DELTA.(t) can then be considered as a histogram as a function of time and the length of stay of the indicating elements (stain molecules or frigories) in the heart, the mean value t of this length of stay can be calculated by the ratio: ##EQU3## by accepting the hypothesis that the molecules of blood have the same length of stay as those of the indicator, the volume V occupied by the blood can be calculated by using the following formula: ##EQU4##
Thus this calculation assumes an instantaneous injection. However, in practice for mechanical reasons it is impossible to reduce its duration to below a considerable fraction of a second, which leads to a considerable error, t normally being of the order of one to three seconds, and moreover due to the mechanical reaction caused by the stream of indicator causes a movement of the catheter so that successive measurements cannot be compared.
It would still be possible to use this process by carrying out a non-instantaneous injection. For this it would be necessary on the basis of a histogram as a function of time and the quantities of indicator injected to calculate an average injection time lag t.sub.o relative to the start of the injection and to substitute in the formula (3): t - t.sub.o to t. The difficulties of carrying out such a process are obvious in that it requires besides the measuring integral of the output the measurement of one or two supplementary integrals making separate and complex calculating devices necessary.
Moreover, in view of rapidity of elimination of the indicator it is obvious that this method is very sensitive to the time of injection relative to the phase of the cardiac cycle, whereby the error for this reason alone can reach 25 percent.